New England Historic Genealogical Society, 101 Newbury St., Boston, MA 02116, 888-296-3447, www.americanancestors.org/store-books (you don't have to be a member to buy their publications)
Be sure to get on the mailing lists of these companies. They will not give your address to anyone else, thus, you will NOT receive junk mail by being on their mailing list.
Many genealogical books are being re-published in CD format. Often, two or more related books are put onto the same CD, sometimes with one combined index. Usually the CD version of books is half (or even less) of the price of the original book.